Thin, non-porous polymer films are acquiring increasing importance as membranes in permeation installations for the selective separation of certain gases or liquids from gas mixtures or liquid mixtures; in reverse osmosis for desalination of brackish water and seawater; in coatings, for example of metallic workpieces; as electrically insulating films in electrical installations and as dielectrics in condenser construction.
Polycarbonate films possess particular importance in these fields because of their unusual properties. For example, because of their very good electrical insulating capacity and high heat distortion point they are used for insulating purposes in electrical plant construction (electric motors, transformers and others) and because of their very good dielectric properties they are used as dielectrics in condenser construction. Because of their high selective permeability they are also proposed for the separation of helium from natural gas and because of their high permeability for oxygen as against nitrogen they are proposed for the enrichment of oxygen in atmospheric air.
To increase the yield of permeation installations, to reduce the constructional size of electrical machinery and to increase the capacity of condensers, it is desirable to manufacture polycarbonate films which are as thin as possible while at the same time preserving the non-porosity, and to use these films for the abovementioned purposes.
The reason for this is that the gas permeability or liquid permeability increases in inverse proportion to the film thickness, that is to say a reduction of the layer thickness of a film to 1/100 of the original layer thickness produces a 100-fold increase in the permeability of the film. Equally, the capacity of condensers increases in inverse proportion to the layer thickness of the dielectric; a reduction in the layer thickness of a film to 1/100 of its original layer thickness thus leads to a 100-fold increase in the capacity of the condenser.
The lowest thickness of commercially manufactured polycarbonate films is currently 2 .mu.m = 2 .times. 10.sup.- .sup.3 mm (sold under the registered trademark MAKROFOL of Bayer AG). Such films are manufactured by casting polycarbonate solutions through slit dies, optionally with subsequent monoaxial, biaxial or surface stretching.
For the abovementioned end uses it is desirable, as indicated above, to manufacture non-porous films based on polycarbonate of which the thickness is approx. one to two powers of ten below the currently achievable minimum thickness of 2 .mu.m, that is to say between 0.01 and 0.2 .mu.m. At these extremely low film thicknesses it will in most cases be necessary to use suitable supporting membranes to ensure the requisite mechanical stability and to mount the polycarbonate films, free of tension, on such carriers. Using the abovementioned currently customary manufacturing processes, however, it is not possible to manufacture the requisite extremely thin polycarbonate films so that they are non-porous and to mount them, if desired, free of tension on carriers.